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Date: Wed, 27 May 2020 16:04:19 +0000 (UTC)
From: Hannes Domani <ssbssa@yahoo.de>
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Subject: Re: [RFC][PATCH 0/6] Step program considering the source column
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X-List-Received-Date: Wed, 27 May 2020 16:04:26 -0000

 Am Mittwoch, 27. Mai 2020, 17:33:37 MESZ hat Tom de Vries <tdevries@suse.d=
e> Folgendes geschrieben:

> On 16-05-2020 19:26, Hannes Domani via Gdb-patches wrote:
> > Basically, this implements the new commands nextc (nc) and stepc (sc),
> > which allow to step through the program until an instruction is reached=
,
> > that belongs to another source column (according to the debug informati=
on).
> >
> > The current column location is visualized in the frame info, and in the=
 TUI.
> >
> > Also, the column information is added to 'maint info line-table' and th=
e
> > python line table interface.
> >
> > Since the frame info output is different, this certainly breaks the
> > testsuite, so this column indicator needs a parameter to disable, but I
> > wasn't concerned about this yet.
> >
> > With the example code from PR25913, it looks like this:
> >
> > (gdb) start
> > Temporary breakpoint 2 at 0x40162d: file gdb-25911.c, line 4.
> > Starting program: C:\src\tests\gdb-25911.exe
> >
> > Temporary breakpoint 2, main () at gdb-25911.c:4
> > 4=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 int a =3D 4;
> >=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0 ^
> > (gdb) nc
> > 6=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 a =3D 5; a =3D 6; a =3D 7;
> >=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 ^
> > (gdb) nc
> > 6=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 a =3D 5; a =3D 6; a =3D 7;
> >=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 ^
> > (gdb) nc
> > 6=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 a =3D 5; a =3D 6; a =3D 7;
> >=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0 ^
> > (gdb) nc
> > 8=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 return 0;
> >
> >
> > What do you think of this so far?
> >
>
> Very nice :)
>
> As I've just learned by looking at this (
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D95360 ) PR, interesting
> code in relation to this patch series is dwarf_record_line_p.
>
> The function implements filtering of line-table elements, and has as
> related test-case gdb.dwarf2/dw2-single-line-discriminators.exp.
>
> If we disable the filtering (by returning 1 at the end instead of 0), we
> get for test-case gdb.dwarf2/dw2-single-line-discriminators.exp:
> ...
> Temporary breakpoint 2, main () at
> gdb.dwarf2/dw2-single-line-discriminators.c:26
> 26=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 x =3D 0;
> (gdb) n
> 28=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 for (i =3D 0; i < 10; ++i) c=
ontinue; /* stepi line */
> (gdb) si
> 28=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 for (i =3D 0; i < 10; ++i) c=
ontinue; /* stepi line */
> ...
>
> That is: no progress is visible after the stepi.=C2=A0 If we re-enable th=
e
> filtering, we have:
> ...
> Temporary breakpoint 1, main () at
> gdb.dwarf2/dw2-single-line-discriminators.c:26
> 26=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 x =3D 0;
> (gdb) n
> 28=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 for (i =3D 0; i < 10; ++i) c=
ontinue; /* stepi line */
> (gdb) si
> 0x00000000004004cd=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 28=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0=C2=A0 for (i =3D 0; i < 10; ++i) continue; /*
> stepi line */
> ...
>
> So, progress is visible.
>
> If we now use your patch series, and with the filtering enabled, we have:
> ...
> Temporary breakpoint 1, main () at
> gdb.dwarf2/dw2-single-line-discriminators.c:26
> 26=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 x =3D 0;
>=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 ^
> (gdb) n
> 28=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 for (i =3D 0; i < 10; ++i) c=
ontinue; /* stepi line */
>=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0 ^
> (gdb) si
> 0x00000000004004cd=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 28=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0=C2=A0 for (i =3D 0; i < 10; ++i) continue; /*
> stepi line */
>=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0 ^
> (gdb)
> 0x00000000004004d1=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 28=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0=C2=A0 for (i =3D 0; i < 10; ++i) continue; /*
> stepi line */
>=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0 ^
> (gdb)
> 0x00000000004004d3=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 28=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0=C2=A0 for (i =3D 0; i < 10; ++i) continue; /*
> stepi line */
>=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0 ^
> (gdb)
> 0x00000000004004d5=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 28=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0=C2=A0 for (i =3D 0; i < 10; ++i) continue; /*
> stepi line */
>=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0 ^
> ...
>
> So, we see progress in the insn addresses, but the column does not progre=
ss.
>
> OTOH, if we disable filtering:
> ...
> Temporary breakpoint 1, main () at
> gdb.dwarf2/dw2-single-line-discriminators.c:26
> 26=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 x =3D 0;
>=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 ^
> (gdb) n
> 28=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 for (i =3D 0; i < 10; ++i) c=
ontinue; /* stepi line */
>=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0 ^
> (gdb) si
> 28=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 for (i =3D 0; i < 10; ++i) c=
ontinue; /* stepi line */
>=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0 ^
> (gdb) si
> 0x00000000004004d1=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 28=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0=C2=A0 for (i =3D 0; i < 10; ++i) continue; /*
> stepi line */
>=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0 ^
> (gdb) si
> 28=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 for (i =3D 0; i < 10; ++i) c=
ontinue; /* stepi line */
>=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 ^
> (gdb) si
> 28=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 for (i =3D 0; i < 10; ++i) c=
ontinue; /* stepi line */
>=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 ^
> (gdb) si
> 0x00000000004004d8=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 28=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0=C2=A0 for (i =3D 0; i < 10; ++i) continue; /*
> stepi line */
>=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0=C2=A0=C2=A0 ^
> ...
> we don't see progress after the first stepi, but eventually we do see
> progress in the column.
>
> Looking at the line-info:
> ...
>=C2=A0=C2=A0 [0x00000147]=C2=A0 Set column to 8
>=C2=A0=C2=A0 [0x00000149]=C2=A0 Special opcode 161: advance Address by 11 =
to 0x4004c6
> and Line by 2 to 28
>=C2=A0=C2=A0 [0x0000014a]=C2=A0 Extended opcode 4: set Discriminator to 4
>=C2=A0=C2=A0 [0x0000014e]=C2=A0 Special opcode 103: advance Address by 7 t=
o 0x4004cd and
> Line by 0 to 28
> ...
> perhaps we we should collapse entries that have the same column, so
> something like this in dwarf_record_line_p:
> ...
>=C2=A0=C2=A0 if (line !=3D last_line)
>=C2=A0=C2=A0=C2=A0=C2=A0 return 1;
>=C2=A0=C2=A0 if (colum !=3D last_column)
>=C2=A0=C2=A0=C2=A0=C2=A0 return 1;
>
>=C2=A0=C2=A0 return 0;
>
> ...

Interesting.
I'm just wondering, if dwarf_record_line_p merges all entries of the same l=
ine,
why does my column-based stepping even work at all?

Is it because of line_has_non_zero_discriminator?
If yes, what is the discriminator?


Hannes